The present invention relates to data transmission circuits and more particularly to a communications medium coupling circuit for continuous coupling of data to a standard telephone line.
Conventional capacitor coupled circuits have an inadequate low frequency response to couple low frequencies onto a telephone line such as those that exist in data-under-voice frequency signals or in certain spread spectrum frequency transmissions.
Conventional transformer coupled circuits generally have a d-c impedance that is too low if not put in series with a d-c blocking capacitor. The capacitor is required to provide a high d-c impedance so that when, for example, a test voltage is applied to the telephone pair, a hundred and fifty volts d-c across tip and ring, the coupling circuit reacts more like an open circuit than a low impedance.
However, when data transmissions are desired which are below voice frequencies the d-c blocking capacitor must be very large and becomes expensive. If the capacitor gets large, however, then its a-c impedance at normal ringing frequencies of about twenty hertz, provides too low an impedance to the ringing current.
Other conventional transistorized coupling circuits in use with telephone sets or for TOUCH-TONE dialers, have a drive impedance that is too low for on-hook ringing conditions and therefore conventionally exist on the equipment side of a switch hook so that during ringing conditions and during testing situations, the coupling circuit is disconnected from the line by the switch hook so that it only has to provide a line matching impedance when the equipment is off-hook.
Consequently, these conventional circuits do not provide an interface circuit which can remain connected to the telephone line at all times for both on-hook and off-hook data transmissions and which can successfully transmit a wide range of frequency signals, including low frequency data signals.
Two transistorized circuits have been devised which can remain connected to the line, one of which is controlled by a microprocessor, the other of which uses a modem attached to the circuit for control. In each, the data transmission is coupled via a coupling transformer and at least one d-c blocking capacitor. Consequently, neither transistorized circuit is suitable for transmission of low frequency data. In addition, each is designed for transmission in only one condition of the telephone line, on-hook or off-hook.
In U.S. Pat. No. 4,203,006 three basic circuit means are used, each controlled by an associated modem which causes data to be transmitted only when the telephone line is in the off-hook condition. A first circuit means detects a ring signal. This is connected to the modem which causes a signal to be generated to indicate that the telephone is off-hook. When the signal is generated to produce an off-hook condition on the telephone line the off-hook current is diverted around a coupling transformer and the modem then sends or receives a-c signals coupled to the telephone line with the use of a coupling transformer and associated blocking capacitor.
In U.S. Pat. No. 4,224,478 a microprocessor controlled coupling circuit is disclosed. The microprocessor periodically pulses the circuit to determine whether or not an associated telephone set is off-hook. When off-hook, data cannot be transmitted. When on-hook the microprocessor is enabled to transmit or receive data, again, coupled to the telephone line with an isolation transformer in series with at least one d-c blocking capacitor.
Consequently, the coupling circuits of both of these inventions enable the transmission of frequency data in a limited frequency range and then only in one operating condition of the telephone line.
These prior coupling circuits are voltage mode devices. Data signals transmitted are superimposed onto whatever d-c voltage is on the line.